I'm a first time cruiser and I'm sailing on carnival paradise which goes to catalina & ensenada. I have not booked any shore excursions. Not sure if need to book through carnival or can i just get down at the port and find some options for the tour. which will be best? any suggestions will be helpful..

Did you check out the "port chat" section further down the page? You can post your questions there, in the "Ports of Call Chat" thread.

Sorry I can't be much help, but I haven't been to either of these ports. Considering it's your first cruise, I would recommend a ship's excursion. Bear in mind the ship will not wait for you if you go out on your own, and are late getting back to the ship. All aboard is around 30 minutes prior to sailing. Example, if it says you're in Catalina until 4:30 pm, you must be back on board by 4:00 pm.

If you go on a ship's excursion, and that excursion is late getting back, they will wait, because they know that passengers are on that excursion.